1. Field of the Invention
The present invention relates to a loop antenna used in an interrogator of a non-contact data carrier system for performing short-distance communication by electromagnetic induction between the interrogator and a transponder. More particularly, the present invention concerns a loop antenna used to send a transmission signal from the interrogator to the transponder and to receive a reply signal from the transponder in the interrogator, which can detect the reply signal from the transponder without detecting the transmission signal emitted from the interrogator, thereby facilitating detection of the reply signal and enhancing reliability of reading.
2. Related Art of the Invention
Receiving attention and quickly spreading these years are non-contact data carrier systems for performing short-distance communication by electromagnetic induction coupling between the interrogator and the transponder. For example, the automatic ticketing system, the access control system, or the like adopts a non-contact information card system using a reader/writer as an interrogator and a card, such as an IC card or an ID card, as a transponder.
In this non-contact data carrier system, in order to send information to the transponder or further to supply power thereto, the interrogator sends electromagnetic waves at frequencies of approximately several hundreds of kHz to several MHz to the transponder and, at the same time, receives the reply signal from the transponder to detect it. In this case, the interrogator is normally provided with a loop antenna as a dual-purpose antenna serving both as a transmitting antenna for sending the signal or the power to the transponder and as a receiving antenna for receiving the reply signal from the transponder.
In the conventional non-contact data carrier systems, however, when the interrogator receives and detects the reply signal from the transponder, at the loop antenna terminals of the interrogator there appear not only the reply signal from the transponder, but also the transmission signal having been sent to the transponder by the interrogator per se. In this case, the longer the communication range (i.e., the distance between the interrogator and the transponder), the higher the intensity of the transmission signal sent from the interrogator to the transponder. Therefore, the ratio of the transmission signal to the reply signal appearing at the loop antenna terminals of the interrogator increases as the communication range becomes longer. Normally, the transmission signal appearing at the loop antenna terminals of the interrogator is about 100 to 100,000 times stronger than the reply signal to be detected. This resulted in a weak reply signal appearing in undesired signals harmful and severe to reception of signal, at the loop antenna terminals of the interrogator. In addition, reception intensities of transmission signals undesirably received upon receiving the reply signal from the transponder at the loop antenna terminals of the interrogator are not constant because of dispersion upon fabrication of loop antennas for interrogator and dispersion in resonance capacitance. Further, since the inductance, capacitance, Q value, and the like of the loop antenna change with a change of temperature, the level of undesired waves also varies every hour at the loop antenna terminals in receiving the reply signal from the transponder. Thus, there was the problem that it was very difficult to receive and accurately detect the reply signal from the transponder by the loop antenna of interrogator and thus not easy to secure the reading reliability of data. This in turn raised the problem that the circuitry for reading the reply signal became complex and expensive.
These problems were commonly observed in the conventional cases, not only in applications where the interrogator was provided with a loop antenna as a dual-purpose antenna serving both as a transmitting antenna and as a receiving antenna,.but also in applications where the interrogator was provided with separate transmitting antenna and receiving antenna. Namely, when the interrogator has the transmitting antenna and the receiving antenna simply separated from each other, coupling occurs between the transmitting antenna and the receiving antenna, whereby the transmission signal from the transmitting antenna is induced directly in the receiving antenna, resulting in undesired transmission signals appearing at high level at the receiving antenna terminals.